The present invention relates to water-in-oil type (hereinafter simply referred to as W/O type) emulsion explosives utilized for industrial blasting operations such as in excavating tunnels, quarrying and mining.
Since the first disclosure by U.S. Pat. No. 3,161,551, various modified inventions have been achieved for the W/O type emulsion explosives such as in U.S. Pat. Nos. 3,242,019, 3,447,978, 3,715,247, 3,770,522, 4,008,108 and 4,482,403, and G.B. Patent No. 1,593,163. The W/O type emulsion explosives in these inventions basically comprise as the continuous phase (oil component), mineral oils, waxes, other hydrophobic carbonaceous fuels, as the discontinuous phase an aqueous solution of oxidizing agent mainly composed of ammonium nitrate and W/O type emulsifier. By further adding optionally sensitizing agents such as nitric acid, strontium ions or hollow microspheres thereto, sensitivity widely ranging from booster initiation to No. 6 cap initiation can be obtained. It has been well known that these W/O type emulsion explosives exhibit high detonating velocity which can not be attained in conventional slurry explosives and are extremely safe as compared with dynamites.
Various improvements have been made for solving the inherent problem of low stability of the W/O type emulsion explosives caused by the nature that an emulsion is prepared by uniformly mixing two or more immiscible liquids with the aid of an emulsifier as shown in Japanese Patent Application Laying Open (KOKAI) No. 56-129694 (1981), U.S. Pat. No. 4,548,660 and U.S. Pat. No. 4,482,403 and, accordingly, the W/O type emulsion explosives have now become to be used widely at present. Since the W/O type emulsion explosives are plastic in their nature, they have advantage not attainable in the conventional slurry explosives that packaging into paper cartridge can be applied with ease and the use of W/O type emulsion explosives is fairly increased at present. However, the W/O type emulsion explosives are softer as compared with dynamite which have been also used as paper cartridges. Accordingly, it has been strongly pointed out in the actual field of using explosives that the W/O type emulsion explosives are readily deformable upon transportation and handling of paper cartridges and deformed paper cartridges of the W/O type emulsion explosive can not easily be charged into charging holes and thus are not convenient for use. In order to improve the strength of the paper cartridge, although the thickness of the cartridge paper may be increased as the countermeasure, this brings about other problems such as increase in the packaging cost or degradation in the blasting fume after the completion of blasting. Although there may be a method of adding powdery additives to increase the hardness of the W/O type emulsion explosives, this method brings about problems such as degradation in the stability of the W/O type emulsion explosives and induces remarkable reduction in the blasting characteristics.
Accordingly, how to increase the hardness of the W/O type emulsion explosives without degrading the stability thereof has been a significant problem to be solved in the art of the W/O type emulsion explosives.
U.S. Pat. No. 4,482,403 discloses an improvement for the pressure resistance of W/O type emulsion explosives by using an emulsifier in which the ratio of sorbide fatty acid ester, sorbitan fatty acid ester and sorbitol fatty acid ester is within a specified range (5-30:5-75:15-90). However, it can not be said that the hardness of the W/O type emulsion explosives using the emulsifier having the above specified mixing ratio of esters in U.S. Pat. No. 4,482,403 is satisfactory.
As a result of the present inventors' studies and experiments for increasing the hardness of W/O type emulsion explosives without degrading the stability thereof, it has been found by the present inventors that the hardness of W/O type emulsion explosives can be significantly increased without degrading the stability thereof by using an emulsifier having an ester mixing ratio not known in the prior art, i.e., by using the ester mixture comprising from 0 to 3% by weight of sorbide fatty acid ester, from 5 to 50% by weight of sorbitan fatty acid ester and from 50 to 95% by weight of sorbitol fatty acid ester, and the present inventors have accomplished the present invention based on the findings.